Semiconductor device and method for manufacturing the same

ABSTRACT

Many of the physical properties of a silicon semiconductor have already been understood, whereas many of the physical properties of an oxide semiconductor have been still unclear. In particular, an adverse effect of an impurity on an oxide semiconductor has been still unclear. In view of the above, a structure is disclosed in which an impurity that influences electrical characteristics of a semiconductor device including an oxide semiconductor layer is prevented or is eliminated. A semiconductor device which includes a gate electrode, an oxide semiconductor layer, and a gate insulating layer provided between the gate electrode and the oxide semiconductor layer and in which the nitrogen concentration in the oxide semiconductor layer is 1×10 20  atoms/cm 3  or less is provided.

TECHNICAL FIELD

The present invention relates to an oxide semiconductor.

BACKGROUND ART

A semiconductor device including an oxide semiconductor is disclosed inPatent Document 1.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2007-123861

DISCLOSURE OF INVENTION

Many of the physical properties of a silicon semiconductor have alreadybeen understood, whereas many of the physical properties of an oxidesemiconductor have been still unclear.

In particular, an adverse effect of an impurity on an oxidesemiconductor has been still unclear.

In view of the above, a structure will be disclosed in which an impuritythat influences electrical characteristics of a semiconductor deviceincluding an oxide semiconductor layer is prevented or is eliminated.

There are two factors of carriers in an oxide semiconductor layer.

The first factor is oxygen deficiency in an oxide semiconductor layer.

The second factor is a donor element or an acceptor element in an oxidesemiconductor layer.

Here, a hydrogen element serves as a carrier (a donor) in an oxidesemiconductor layer.

Further, since a hydrogen element has a reducing character, it alsoserves as an element causing oxygen deficiency.

Therefore, it is said that a substance containing a hydrogen element isan element which prevents an oxide semiconductor layer from being highlypurified so that the oxide semiconductor layer is not close to an i-typeoxide semiconductor layer because a hydrogen element has two factors ofinducing carriers.

Note that as a substance containing a hydrogen element, for example,hydrogen, moisture, hydroxide, hydride, and the like can be given.

As a result of research, the present inventors found that, surprisingly,hydrogen is more likely to enter an oxide semiconductor layer when alarge amount of nitrogen is contained in the oxide semiconductor layer.

Conversely, hydrogen is less likely to enter an oxide semiconductorlayer in which the nitrogen concentration is reduced.

Specifically, the nitrogen concentration in an oxide semiconductor layermeasured by secondary ion mass spectrometry (SIMS) is set to 1×10²⁰atoms/cm³ or less (or less than 1×10²⁰ atoms/cm³), whereby an oxidesemiconductor layer which is difficult for hydrogen to enter can beformed.

That is, a semiconductor device which includes a gate electrode, anoxide semiconductor layer, and a gate insulating layer provided betweenthe gate electrode and the oxide semiconductor layer and in which thenitrogen concentration in the oxide semiconductor layer is 1×10²⁰atoms/cm³ or less can be provided.

A semiconductor device which includes a gate electrode, a gateinsulating layer provided over the gate electrode, an oxidesemiconductor layer provided over the gate insulating layer, and a pairof contact electrodes provided over the oxide semiconductor layer and inwhich the nitrogen concentration in the oxide semiconductor layer is1×10²⁰ atoms/cm³ or less can be provided.

A semiconductor device which includes a gate electrode, a gateinsulating layer provided over the gate electrode, a pair of contactelectrodes provided over the gate insulating layer, and an oxidesemiconductor layer provided over the gate insulating layer and the pairof contact electrodes and in which the nitrogen concentration in theoxide semiconductor layer is 1×10²⁰ atoms/cm³ or less can be provided.

A semiconductor device which includes a gate electrode, a gateinsulating layer provided over the gate electrode, an oxidesemiconductor layer provided over the gate insulating layer, a channelprotective layer provided over the oxide semiconductor layer, and a pairof contact electrodes provided over the oxide semiconductor layer andthe channel protective layer and in which the nitrogen concentration inthe oxide semiconductor layer is 1×10²⁰ atoms/cm³ or less can beprovided.

In addition, the semiconductor device in which the hydrogenconcentration in the oxide semiconductor layer is 6×10¹⁸ atoms/cm³ orless can be provided.

A method for manufacturing a semiconductor device including a gateelectrode, an oxide semiconductor layer, and a gate insulating layerprovided between the gate electrode and the oxide semiconductor layer,which includes the step of performing heat treatment on the oxidesemiconductor layer having a nitrogen concentration of 1×10²⁰ atoms/cm³or less at 350° C. or higher for 1 hour or longer, can be provided.

A method for manufacturing a semiconductor device including a gateelectrode, an oxide semiconductor layer, and a gate insulating layerprovided between the gate electrode and the oxide semiconductor layer,which includes the step of performing heat treatment on the oxidesemiconductor layer having a nitrogen concentration of 1×10²⁰ atoms/cm³or less at 450° C. or higher for 1 hour or longer, can be provided.

A method for manufacturing a semiconductor device including a gateelectrode, an oxide semiconductor layer, and a gate insulating layerprovided between the gate electrode and the oxide semiconductor layer,which includes the step of performing heat treatment on the oxidesemiconductor layer having a nitrogen concentration of 1×10²⁰ atoms/cm³or less at 550° C. or higher for 1 hour or longer, can be provided.

A method for manufacturing a semiconductor device including a gateelectrode, an oxide semiconductor layer, and a gate insulating layerprovided between the gate electrode and the oxide semiconductor layer,which includes the step of performing heat treatment on the oxidesemiconductor layer having a nitrogen concentration of 1×10²⁰ atoms/cm³or less at 650° C. or higher for 3 minutes or longer, can be provided.

The nitrogen concentration in an oxide semiconductor layer is reduced,whereby an oxide semiconductor layer which is difficult for hydrogen toenter can be formed.

In other words, the nitrogen concentration in an oxide semiconductorlayer is reduced, whereby hydrogen can be prevented from entering theoxide semiconductor layer.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C show an example of a method for manufacturing asemiconductor device.

FIGS. 2A and 2B show an example of a method for manufacturing asemiconductor device.

FIGS. 3A and 3B show an example of a method for manufacturing asemiconductor device.

FIGS. 4A to 4C each show an example of a semiconductor device.

FIGS. 5A to 5C show an example of a method for manufacturing asemiconductor device.

FIGS. 6A and 6B show an example of a method for manufacturing asemiconductor device.

FIGS. 7A to 7C show an example of a method for manufacturing asemiconductor device.

FIGS. 8A and 8B show an example of a method for manufacturing asemiconductor device.

FIG. 9 shows an example of a sputtering apparatus.

FIGS. 10A and 10B each show SIMS data.

FIGS. 11A and 11B each show SIMS data.

FIGS. 12A and 12B each show SIMS data.

FIGS. 13A and 13B each show SIMS data.

FIGS. 14A and 14B each show TDS data.

FIGS. 15A and 15B each show an example of a semiconductor device.

FIG. 16 shows a TEM photograph.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments and examples will be described in detail with reference tothe accompanying drawings.

It is easily understood by those skilled in the art that modes anddetails of the present invention can be modified in various ways withoutdeparting from the spirit of the present invention.

Therefore, the present invention should not be interpreted as beinglimited to the description of the embodiments and examples below.

Note that in the structures described below, the same portions orportions having similar functions are denoted by the same referencenumerals in different drawings, and explanation thereof will not berepeated.

The following embodiments and examples can be combined with each other,as appropriate.

Embodiment 1

An example of a method for manufacturing a semiconductor device will bedescribed.

First, a gate electrode 200 is formed over a substrate 100 having aninsulating surface. Then, a gate insulating layer 300 is formed over thegate electrode 200, and an oxide semiconductor layer 400 is formed overthe gate insulating layer 300 (FIG. 1A).

As the substrate, any material can be used. For example, a glasssubstrate, a quartz substrate, a metal substrate, a plastic substrate,or a semiconductor substrate can be used, but the substrate is notlimited to these examples.

In the case where an insulating substrate is used as the substrate, thesubstrate has an insulating surface.

On the other hand, in the case where a metal substrate, a semiconductorsubstrate, or the like is used as the substrate, the substrate can havean insulating surface when a base insulating layer is formed over thesubstrate.

Note that a base insulating layer may be formed over the substrate alsoin the case where an insulating substrate is used as the substrate.

As the gate electrode, any material having conductivity can be used. Forexample, aluminum, titanium, molybdenum, tungsten, gold, silver, copper,silicon, a variety of alloys, or an oxide conductive layer (typically,indium tin oxide and the like) can be used, but the gate electrode isnot limited to these examples. The gate electrode may have asingle-layer structure or a stacked-layer structure.

As the gate insulating layer, any material having an insulating propertycan be used. For example, a silicon oxide film, a silicon nitride film,a silicon oxide film containing nitrogen, a silicon nitride filmcontaining oxygen, an aluminum nitride film, an aluminum oxide film, ahafnium oxide film, or the like can be used, but the gate insulatinglayer is not limited to these examples. The gate insulating layer mayhave a single-layer structure or a stacked-layer structure.

Note that the amount of hydrogen and nitrogen in the gate insulatinglayer is preferably small so that injection of carriers into the oxidesemiconductor layer can be prevented.

The gate insulating layer in which the amount of hydrogen is small ispreferably formed using a deposition gas which does not contain hydrogen(H) or hydride (e.g., SiH₄).

The gate insulating layer in which the amount of nitrogen is small ispreferably formed using a deposition gas which does not contain nitrogen(N) or nitride (e.g., N₂O or NH₄).

Therefore, when attention is focused on the point that the amount ofhydrogen is small, a gate insulating film formed by a sputtering methodis preferably used because hydride (e.g., SiH₄) is used in a plasma CVDmethod.

Further, when attention is focused on the point that the amount ofnitrogen is small, an oxide film which does not contain nitrogen ispreferably used.

Note that a gate insulating layer formed by a plasma CVD method hasfewer defects and has higher film quality than a gate insulating layerformed by a sputtering method.

Therefore, in some cases, transistor characteristics become favorablewhen a gate insulating layer formed by a plasma CVD method is used.

Thus, a plasma CVD method, a sputtering method, or another method may beused as appropriate as needed.

Note that in the case where a gate insulating layer formed by a plasmaCVD method is used, a substance containing a hydrogen element iseliminated from the gate insulating layer when heat treatment isperformed; therefore, in the case where a plasma CVD method is used,heat treatment (at higher than or equal to 200° C. and lower than orequal to 1000° C. (preferably, higher than or equal to 300° C. and lowerthan or equal to 800° C.)) is preferably performed after the gateinsulating layer is formed.

Note that as a substance containing a hydrogen element, for example,hydrogen, moisture, hydroxide, hydride, and the like can be given.

Examples of the oxide semiconductor layer include, but not limited to,In—Ga—Zn—O-based oxide (containing indium, gallium, zinc, and oxygen asthe main components), In—Sn—Zn—O-based oxide (containing indium, tin,zinc, and oxygen as the main components), In—Al—Zn—O-based oxide(containing indium, aluminum, zinc, and oxygen as the main components),Sn—Ga—Zn—O-based oxide (containing tin, gallium, zinc, and oxygen as themain components), Al—Ga—Zn—O-based oxide (containing aluminum, gallium,zinc, and oxygen as the main components), Sn—Al—Zn—O-based oxide(containing tin, aluminum, zinc, and oxygen as the main components),In—Zn—O-based oxide (containing indium, zinc, and oxygen as the maincomponents), Sn—Zn—O-based oxide (containing tin, zinc, and oxygen asthe main components), Al—Zn—O-based oxide (containing aluminum, zinc,and oxygen as the main components), In—O-based oxide (oxide of indium(indium oxide)), Sn—O-based oxide (oxide of tin (tin oxide)), Zn—O-basedoxide (oxide of zinc (zinc oxide)), and the like.

The oxide semiconductor layer can be formed by a sputtering method, anevaporation method, or the like, for example.

The thickness of the oxide semiconductor layer is preferably 5 nm to 1μm (more preferably, 20 nm to 80 nm).

In the case where the oxide semiconductor layer is formed, carefulattention needs to be paid so that nitrogen is not contained in theoxide semiconductor layer.

Specifically, the oxide semiconductor layer is preferably formed so thatthe nitrogen concentration in the oxide semiconductor layer measured bysecondary ion mass spectrometry (SIMS) is 1×10²⁰ atoms/cm³ or less (orless than 1×10²⁰ atoms/cm³), 5×10¹⁹ atoms/cm³ or less (or less than5×10¹⁹ atoms/cm³), 1×10¹⁹ atoms/cm³ or less (or less than 1×10¹⁹atoms/cm³), 5×10¹⁸ atoms/cm³ or less (or less than 5×10¹⁸ atoms/cm³), or1×10¹⁸ atoms/cm³ or less (or less than 1×10¹⁸ atoms/cm³).

Note that nitrogen is less likely to enter the oxide semiconductor layereven in the case where the oxide semiconductor layer is subjected toheat treatment.

Therefore, the nitrogen concentration in the oxide semiconductor layerafter the semiconductor device is completed, which is measured bysecondary ion mass spectrometry (SIMS), is also preferably 1×10²⁰atoms/cm³ or less (or less than 1×10²⁰ atoms/cm³), 5×10¹⁹ atoms/cm³ orless (or less than 5×10¹⁹ atoms/cm³), 1×10¹⁹ atoms/cm³ or less (or lessthan 1×10¹⁹ atoms/cm³), 5×10¹⁸ atoms/cm³ or less (or less than 5×10¹⁸atoms/cm³), or 1×10¹⁸ atoms/cm³ or less (or less than 1×10¹⁸ atoms/cm³).

Note that as the value of the nitrogen concentration, the average valuein the effective range of secondary ion mass spectrometry (SIMS) can beadopted.

Alternatively, as the value of the nitrogen concentration, the maximumvalue in the effective range of secondary ion mass spectrometry (SIMS)may be adopted (when the maximum value in the effective range is smallerthan a predetermined value, the average value in the effective range isalso smaller than the predetermined value).

Here, an example of a sputtering apparatus is shown in FIG. 9.

The sputtering apparatus in FIG. 9 includes a deposition chamber 2001, acover 2002, a target 2003, and a pump 2004.

Note that a structure is employed in which a substrate is put to beprovided for the cover 2002 and the target 2003 is sputtered fordeposition.

The deposition chamber 2001 and the cover 2002 are connected to eachother, and an O-ring 2005 is provided for a connecting portion.

The deposition chamber 2001 and the target 2003 are connected to eachother, and an O-ring 2006 is provided for a connecting portion.

The deposition chamber 2001 and the pump 2004 are connected to eachother, and a metal gasket 2007 and a metal gasket 2008 are provided forconnecting portions.

Both the O-rings and the metal gaskets are used to prevent leakage fromthe connecting portion. In other words, by the O-rings and the metalgaskets, air (in particular, nitrogen) is prevented from entering thedeposition chamber 2001.

The O-ring is a ring-like packing. A material thereof is, for example,rubber.

The metal gasket is a ring-like fixing sealant. A material thereof is,for example, metal.

The cover 2002 and the target 2003 are frequently opened and closed;therefore, the O-ring is used because it can be easily put on and takenoff.

On the other hand, the pump is hardly opened and closed. Although themetal gasket is not easily put on and taken off, the metal gasket isprovided for the pump because it can improve airtightness compared withthe O-ring.

In the case where the O-ring or the metal gasket has a chip, a crack, orthe like, leakage occurs, so that air outside the deposition chamber2001 enters the deposition chamber 2001.

Since air contains a large amount of nitrogen, careful attention needsto be paid so that the O-ring or the metal gasket does not have a chip,a crack, or the like in order to prevent air (in particular, nitrogen)due to the chip, the crack, or the like of the O-ring or the metalgasket from entering the deposition chamber 2001.

Although the maintenance needs to be further done, it is effective toreplace all the O-rings with metal gaskets which can improveairtightness because air (in particular, nitrogen) can be prevented fromentering the deposition chamber 2001.

On the other hand, in the case where nitrogen is attached to or entersthe inner wall of the deposition chamber 2001 or a surface of the target2003 or in the case where nitrogen floats in the deposition chamber 2001even when air (in particular, nitrogen) is prevented from entering thedeposition chamber 2001, nitrogen might enter an oxide semiconductorlayer.

Thus, in order to remove nitrogen which is attached to or enters theinner wall of the deposition chamber, the deposition chamber is heatedat higher than or equal to 200° C. and lower than or equal to 500° C.

The deposition chamber is heated, whereby nitrogen which is attached toor enters the inner wall of the deposition chamber is released into thedeposition chamber.

Then, nitrogen which is released through heating of the depositionchamber is evacuated with the use of the pump 2004, whereby nitrogenwhich is attached to or enters the inner wall of the deposition chambercan be removed.

Further, after the heat treatment and the evacuation treatment areperformed on the deposition chamber and before the oxide semiconductorlayer used for a semiconductor device is formed, a step of forming anoxide semiconductor layer over a dummy substrate is preferablyperformed.

When the step of forming the oxide semiconductor layer over the dummysubstrate is performed, nitrogen which is attached to or enters thesurface of the target can be removed.

Further, since nitrogen which remains in the deposition chamber due tothe step of forming the oxide semiconductor layer over the dummysubstrate is taken into the oxide semiconductor layer over the dummysubstrate, nitrogen which remains in the deposition chamber can beremoved before the oxide semiconductor layer used for the semiconductordevice is formed.

After that, the dummy substrate is taken out of the deposition chamber,and the oxide semiconductor layer used for the semiconductor device isformed.

Note that it is effective to perform deposition onto the dummy substratea plurality of times.

As described above, the reduction of leakage from the depositionchamber, the reduction of nitrogen on the inner wall of the depositionchamber, deposition onto the dummy substrate, or the like issufficiently performed, so that incorporation of nitrogen into the oxidesemiconductor layer can be thoroughly prevented.

Next, the oxide semiconductor layer 400 is etched into an island shapeby a photolithography method, whereby an oxide semiconductor layer 410is formed (FIG. 1B).

Next, the oxide semiconductor layer is subjected to first heat treatment(at higher than or equal to X° C. and lower than Y° C.).

A nitrogen atmosphere, a rare gas atmosphere, an oxygen atmosphere, anatmosphere containing oxygen and nitrogen, an atmosphere containingoxygen and a rare gas, an atmosphere containing nitrogen and a rare gas,an atmosphere containing oxygen, nitrogen, and a rare gas, or the likecan be selected as appropriate for the atmosphere in which the firstheat treatment is performed.

The first heat treatment may be performed before the oxide semiconductorlayer 400 is etched into an island shape to form the oxide semiconductorlayer 410.

On the other hand, through the step of etching the oxide semiconductorlayer 400 into an island shape to form the oxide semiconductor layer 410by a photolithography method, the oxide semiconductor layer is immersedin moisture of a photoresist and a stripping solution.

Therefore, in order to remove moisture due to the photoresist and thestripping solution, it is more preferable that the first heat treatmentbe performed after the oxide semiconductor layer 400 is etched into anisland shape to form the oxide semiconductor layer 410.

The lower limit (X° C.) of the temperature of the first heat treatmentcan be set to 350° C. or higher (or higher than 350° C.), 400° C. orhigher (or higher than 400° C.), 450° C. or higher (or higher than 450°C.), 500° C. or higher (or higher than 500° C.), 550° C. or higher (orhigher than 550° C.), 600° C. or higher (or higher than 600° C.), 650°C. or higher (or higher than 650° C.), 700° C. or higher (or higher than700° C.), or 750° C. or higher (or higher than 750° C.).

The first heat treatment is preferably performed by a heating methodwith the use of a furnace, an oven, gas RTA, or the like.

Gas RTA refers to a method in which an object is put in a gas heated athigh temperature for a short time (several minutes to several tens ofminutes) to be rapidly heated.

There is no particular limitation on the upper limit of the temperatureof the first heat treatment because the temperature of the first heattreatment is preferably high.

However, the upper limit (Y° C.) of the temperature of the first heattreatment is preferably lower than the allowable temperature limit ofthe substrate.

Further, the upper limit (Y° C.) of the temperature of the first heattreatment can be set to 1000° C. or lower (or lower than 1000° C.), 900°C. or lower (or lower than 900° C.), 800° C. or lower (or lower than800° C.), or 700° C. or lower (or lower than 700° C.).

The heating time of the first heat treatment is preferably 1 hour orlonger. The upper limit of the heating time is not particularly limited,but can be set to 10 hours or shorter, 9 hours or shorter, or 8 hours orshorter in consideration of reduction in process time.

In the case where the first heat treatment is performed with the use ofgas RTA, the heating time of the first heat treatment is preferably 3minutes or longer. The upper limit of the heating time is notparticularly limited, but can be set to 1 hour or shorter, 50 minutes orshorter, or 40 minutes or shorter.

When the measurement is performed by thermal desorption spectroscopy(TDS), a sample on which baking has been performed at 450° C. for 1 hourdoes not have a peak of moisture at around 300° C. A sample on whichbaking has been performed at 650° C. for 3 minutes with the use of gasRTA also does not have a peak of moisture at around 300° C. On the otherhand, a sample on which baking has been performed at 350° C. for 1 hourhas a peak of moisture at around 300° C.

When the measurement is performed by secondary ion mass spectrometry(SIMS), the hydrogen concentration in a sample on which baking has beenperformed at 550° C. for 1 hour is lower than that in a sample on whichbaking has been performed at 450° C. for 1 hour by almost 1 digit.

In other words, the oxide semiconductor layer in which the nitrogenconcentration is reduced is subjected to the first heat treatment undera predetermined condition, whereby substances containing a hydrogenelement which adversely affect electrical characteristics of atransistor can be drastically reduced.

Note that the higher the energy which is applied to the oxidesemiconductor is, the more substances containing a hydrogen element arelikely to be eliminated; therefore, the heating temperature ispreferably high and the heating time is preferably long.

The hydrogen concentration in the oxide semiconductor layer after thefirst heat treatment is preferably 6×10¹⁸ atoms/cm³ or less (or lessthan 6×10¹⁸ atoms/cm³), 5×10¹⁸ atoms/cm³ or less (or less than 5×10¹⁸atoms/cm³), 4×10¹⁸ atoms/cm³ or less (or less than 4×10¹⁸ atoms/cm³),3×10¹⁸ atoms/cm³ or less (or less than 3×10¹⁸ atoms/cm³), 1×10¹⁶atoms/cm³ or less (or less than 1×10¹⁶ atoms/cm³), 1×10¹⁴ atoms/cm³ orless (or less than 1×10¹⁴ atoms/cm³), or 1×10¹² atoms/cm³ or less (orless than 1×10¹² atoms/cm³).

The oxide semiconductor layer in which the nitrogen concentration isreduced can prevent incorporation of hydrogen in steps after the firstheat treatment.

Thus, the hydrogen concentration in the oxide semiconductor layer afterthe semiconductor device is completed is also preferably 6×10¹⁸atoms/cm³ or less (or less than 6×10¹⁸ atoms/cm³), 5×10¹⁸ atoms/cm³ orless (or less than 5×10¹⁸ atoms/cm³), 4×10¹⁸ atoms/cm³ or less (or lessthan 4×10¹⁸ atoms/cm³), 3×10¹⁸ atoms/cm³ or less (or less than 3×10¹⁸atoms/cm³), 1×10¹⁶ atoms/cm³ or less (or less than 1×10¹⁶ atoms/cm³),1×10¹⁴ atoms/cm³ or less (or less than 1×10¹⁴ atoms/cm³), or 1×10¹²atoms/cm³ or less (or less than 1×10¹² atoms/cm³).

Note that as the value of the hydrogen concentration, the average valuein the effective range of secondary ion mass spectrometry (SIMS) can beadopted.

Alternatively, as the value of the hydrogen concentration, the maximumvalue in the effective range of secondary ion mass spectrometry (SIMS)may be adopted (when the maximum value in the effective range is smallerthan a predetermined value, the average value in the effective range isalso smaller than the predetermined value).

Note that the smaller the quantity of substances containing a hydrogenelement in the oxide semiconductor layer is, the more electricalcharacteristics of the transistor including the oxide semiconductorlayer are improved.

Next, a conductive layer 500 is formed over the oxide semiconductorlayer 410 (FIG. 1C).

As the conductive layer, any material having conductivity can be used.For example, aluminum, titanium, molybdenum, tungsten, yttrium, indium,gold, silver, copper, silicon, a variety of alloys containing any ofthese metals, an oxide conductive layer (typically, indium tin oxide),or the like can be used, but the conductive layer is not limited tothese examples. The conductive layer may have a single-layer structureor a stacked-layer structure.

Note that the conductive layer in contact with the oxide semiconductorlayer is formed using titanium, indium, yttrium, an indium-zinc alloy,an alloy containing gallium (e.g., gallium nitride), or the like,whereby the contact resistance between the oxide semiconductor layer andan electrode (a wiring) which is formed by etching of the conductivelayer can be reduced.

The reason why the contact resistance can be reduced is that theelectron affinity of titanium, indium, yttrium, an indium-zinc alloy, analloy containing gallium (e.g., gallium nitride), or the like is lowerthan that of the oxide semiconductor layer.

That is, in the case of a single layer, a metal (or an alloy, acompound) having lower electron affinity than the oxide semiconductorlayer is preferable.

On the other hand, in the case of a stacked layer, a metal (or an alloy,a compound) having lower electron affinity than the oxide semiconductorlayer is preferably disposed to be in contact with the oxidesemiconductor layer.

Since titanium (Ti), indium (In), yttrium (Y), an indium-zinc alloy (anIn—Zn alloy), an alloy containing gallium (Ga) (e.g., gallium nitride),or the like has high resistivity, a material having low resistivity suchas aluminum (Al), gold (Au), silver (Ag), copper (Cu), or a variety ofalloys containing any of these metals is preferably stacked over theconductive layer which is disposed to be in contact with the oxidesemiconductor layer.

Specifically, the following examples can be given, but not limited to, astructure in which Ti and Al are sequentially stacked, a structure inwhich Ti and an alloy containing Al are sequentially stacked, astructure in which Y and Al are sequentially stacked, a structure inwhich Y and an alloy containing Al are sequentially stacked, a structurein which Ti, Al, and Ti are sequentially stacked, a structure in whichTi, an alloy containing Al, and Ti are sequentially stacked, a structurein which In, Al, and Mo are sequentially stacked, a structure in whichY, Al, and Ti are sequentially stacked, a structure in which Mo, Al, andTi are sequentially stacked, and a structure in which Ti, an alloycontaining Al, Mo, and Ti are sequentially stacked.

Note that the alloy having low resistivity refers to an alloy containingaluminum, gold, silver, copper, or the like and another substance (e.g.,Al—Si, Al—Ti, Al—Nd, Cu—Pb—Fe, or Cu—Ni).

Note that the oxide conductive layer can be formed using a materialsimilar to the material of the oxide semiconductor layer.

There is no particular limitation on the oxide conductive layer as longas the resistivity of the oxide conductive layer is lower than that ofthe oxide semiconductor layer serving as a channel formation region.

Here, the oxide conductive layer is oxide which is intentionally made tocontain a large quantity of substances containing a hydrogen element ora large quantity of oxygen deficiency. Substances containing a hydrogenelement and oxygen deficiency induce carriers, so that the conductivityof the oxide can be raised.

The oxide semiconductor layer is oxide which is intentionally made notto contain substances containing a hydrogen element or oxygendeficiency.

That is, the quantity of substances containing a hydrogen element oroxygen deficiency in the oxide semiconductor layer is controlled,whereby the resistivity can be controlled.

Note that in the case where the oxide conductive layer and the oxidesemiconductor layer serving as a channel formation region are formedusing different materials and the oxide conductive layer has lowerresistivity than the oxide semiconductor layer serving as a channelformation region, the resistivity does not need to be controlled bycontrol of the quantity of substances containing a hydrogen element oroxygen deficiency in the oxide semiconductor layer.

Next, the conductive layer 500 is etched to form a plurality ofelectrodes or a plurality of wirings (a source electrode (a contactelectrode), a drain electrode (a contact electrode), a wiring, and thelike) (FIG. 2A). Note that in FIG. 2A, a contact electrode 510, acontact electrode 520, and the like are shown.

By the step of FIG. 2A, a transistor (a channel-etched transistor) iscompleted.

Note that a portion surrounded by a dashed line 8000 in FIGS. 2A and 2Bis slightly etched at the time of etching the conductive layer 500.

The portion surrounded by the dashed line 8000 is called a back channelbecause it is disposed on the rear side of the channel formation region.

Next, an insulating layer 600 (a protective layer or an interlayerinsulating film) is formed to cover the transistor (FIG. 2B).

As the insulating layer, any material having an insulating property canbe used. For example, a silicon oxide film, a silicon nitride film, asilicon oxide film containing nitrogen, a silicon nitride filmcontaining oxygen, an aluminum nitride film, an aluminum oxide film, asiloxane film, an acrylic film, a polyimide film, or the like can beused, but the insulating layer is not limited to these examples. Theinterlayer insulating film may have a single-layer structure or astacked-layer structure.

Here, the kind of insulating layer is changed to compare electricalcharacteristics of transistors. It is found that a film which is formedwithout using a substance containing a hydrogen element as a sputteringgas is preferably used for the insulating layer in a portion in contactwith the back channel (the portion surrounded by the dashed line 8000).

When a substance containing a hydrogen element is contained in the backchannel, the threshold voltage (Vth) of the transistor shifts in anegative direction.

Since a deposition gas containing a hydrogen element (typically, SiH₄ orthe like) is used in a plasma CVD method, a substance containing ahydrogen element is added to the back channel when the insulating layeris formed by a plasma CVD method.

A siloxane film, an acrylic film, a polyimide film, or the like containsa large amount of moisture; therefore, a substance containing a hydrogenelement is constantly supplied to the back channel if any of these filmsis employed.

That is, a film in which the quantity of substances containing ahydrogen element is small is preferably used for the insulating layer incontact with the back channel.

Note that the step of FIG. 2B corresponds to a step of forming theinsulating layer 600 over the contact electrode 510, the contactelectrode 520, and the back channel (the portion surrounded by thedashed line 8000).

A structure may be employed in which, after the step of FIG. 2B, contactholes are formed in the insulating layer 600 and a wiring 810, a wiring820, and the like are formed over the insulating layer 600 (FIG. 3A).

Alternatively, a structure may be employed in which, after the step ofFIG. 2B, a contact hole is formed in the insulating layer 600 and apixel electrode 910 is formed over the insulating layer 600 (FIG. 3B).

After the wirings are formed over the insulating layer 600 asillustrated in FIG. 3A, an insulating layer, a wiring, a transistor, adisplay element, an antenna, or the like may be further formed over thewirings.

After the pixel electrode 910 is formed as illustrated in FIG. 3B, adisplay element (e.g., an EL element or a liquid crystal element) isformed, whereby a display device can be formed.

After the step of FIG. 3A or FIG. 3B, second heat treatment ispreferably performed.

The second heat treatment may be performed between the step of FIG. 2Band the step of FIG. 3A or FIG. 3B.

In other words, the timing of the second heat treatment is notparticularly limited as long as it is performed after the insulatinglayer 600 is formed.

The second heat treatment is preferably performed at higher than orequal to 150° C. and lower than or equal to 500° C. (preferably higherthan or equal to 200° C. and lower than or equal to 300° C.).

The heating time of the second heat treatment is preferably longer thanor equal to 1 hour and shorter than or equal to 10 hours.

The second heat treatment is preferably performed using a furnace, anoven, gas RTA, or the like.

Note that hydrogen in the oxide semiconductor layer is released andoxygen in the oxide semiconductor layer is also released by the firstheat treatment which has been performed previously.

That is, by the first heat treatment, oxygen deficiency is formed in theoxide semiconductor layer.

Thus, when the second heat treatment is performed, the insulating layeris made to be in an oxygen-excess state; therefore, oxygen can besupplied to the oxide semiconductor layer and oxygen deficiency in theoxide semiconductor layer can be reduced.

Examples of the formation method of the insulating layer containingexcessive oxygen include, but are not limited to, a method in which, inthe case where a non-oxide target (silicon, aluminum, or the like) isused as a sputtering target and oxygen is used as a sputtering gas forreactive sputtering, the flow rate of oxygen is increased; a method inwhich an oxide target (silicon oxide, aluminum oxide, or the like) isused as a sputtering target and oxygen is used as a sputtering gas (inthe case where an oxide target is used, oxygen is not used as asputtering gas in general); and a method in which an insulating layer isformed and oxygen is introduced into the insulating layer by ionimplantation or ion doping (note that in the case where reactivesputtering is performed, a gas such as argon is not preferably used sothat a sputtering gas contains oxygen at 100%).

That is, a method in which oxygen is used as a deposition gas forformation of the insulating layer, a method in which oxygen is added tothe insulating layer after the insulating layer is formed, or the likemay be used. Needless to say, oxygen may be used as a deposition gas forformation of the insulating layer, and in addition, oxygen may be addedto the insulating layer after the insulating layer is formed.

Note that in the case where titanium is used for the contact electrode,the second heat treatment is performed after the insulating layer 600 isformed, so that titanium oxide can be formed between the oxidesemiconductor layer and titanium.

By formation of titanium oxide, the contact resistance between the oxidesemiconductor layer and titanium can be reduced.

Note that when titanium oxide is present between the oxide semiconductorlayer and titanium, the contact resistance between the oxidesemiconductor layer and titanium can be reduced, so that a structure maybe formed in which titanium oxide and titanium are sequentially stackedwhen the contact electrode is formed.

In this case, titanium oxide can be formed by a sputtering method, anevaporation method, or the like.

The contents of this embodiment or part thereof can be implemented incombination with any of the other embodiments and examples.

Embodiment 2

In this embodiment, a semiconductor device including a transistor whichhas a structure different from that of the transistor in Embodiment 1will be described.

Note that materials or the like of the layers are the same as those inEmbodiment 1.

A transistor in FIG. 4A is a bottom-gate bottom-contact (BGBC)transistor, and includes the gate electrode 200 provided over thesubstrate 100 having an insulating surface; the gate insulating layer300 provided over the gate electrode 200; the contact electrode 510 andthe contact electrode 520 provided over the gate insulating layer 300;and the oxide semiconductor layer 410 (island shape) provided over thegate insulating layer 300, the contact electrode 510, and the contactelectrode 520.

Note that the insulating layer 600 is provided to cover the transistor.

A portion surrounded by the dashed line 8000 serves as a back channel.

A transistor in FIG. 4B is a top-gate transistor, and includes the oxidesemiconductor layer 410 (island shape) provided over the substrate 100having an insulating surface; the gate insulating layer 300 providedover the oxide semiconductor layer 410; and the gate electrode 200provided over the gate insulating layer 300.

Note that the insulating layer 600 is provided to cover the transistor,and the wiring 810, the wiring 820, and a wiring 830 are providedthrough contact holes provided in the insulating layer 600.

A transistor in FIG. 4C is a channel-stop transistor, and includes thegate electrode 200 provided over the substrate 100 having an insulatingsurface; the gate insulating layer 300 provided over the gate electrode200; the oxide semiconductor layer 410 (island shape) provided over thegate insulating layer 300; a channel protective layer 700 provided overthe oxide semiconductor layer 410; and the contact electrode 510 and thecontact electrode 520 provided over the oxide semiconductor layer 410and the channel protective layer 700.

Note that the insulating layer 600 is provided to cover the transistor.

A portion surrounded by the dashed line 8000 serves as a back channel.

Here, the channel protective layer 700 can be formed using a materialsimilar to the material of the insulating layer 600 described inEmbodiment 1. The channel protective layer 700 and the insulating layer600 may be formed using either the same material or different materials.

In the channel-stop transistor, a portion in contact with the backchannel is not the insulating layer 600 but the channel protective layer700.

That is, a film in which the quantity of substances containing ahydrogen element is small is preferably used for the channel protectivelayer 700.

A transistor in FIG. 15A is a top-gate bottom-contact (TGBC) transistor,and includes the contact electrode 510 and the contact electrode 520provided over a base insulating layer 900; the oxide semiconductor layer410 (island shape) provided over the base insulating layer 900, thecontact electrode 510, and the contact electrode 520; the gateinsulating layer 300 provided over the oxide semiconductor layer 410;and the gate electrode 200 provided over the gate insulating layer 300.

Note that the base insulating layer 900 is provided over the substrate100.

The insulating layer 600 is provided to cover the transistor.

A portion surrounded by the dashed line 8000 serves as a back channel.

Here, the base insulating layer 900 can be formed using a materialsimilar to the material of the insulating layer 600 described inEmbodiment 1. The base insulating layer 900 and the insulating layer 600may be formed using either the same material or different materials.

In the top-gate bottom-contact transistor, a portion in contact with theback channel is not the insulating layer 600 but the base insulatinglayer 900.

That is, a film in which the quantity of substances containing ahydrogen element is small is preferably used for the base insulatinglayer 900.

The base insulating layer 900 is preferably formed using an insulatinglayer containing excessive oxygen by a method similar to the methoddescribed in Embodiment 1. In this case, when first heat treatment isperformed, oxygen is released from the oxide semiconductor layer andoxygen is supplied to the oxide semiconductor layer from the baseinsulating layer at the same time.

A transistor in FIG. 15B is a top-gate top-contact (TGTC) transistor,and includes the oxide semiconductor layer 410 (island shape) providedover the base insulating layer 900; the contact electrode 510 and thecontact electrode 520 provided over the oxide semiconductor layer 410and the base insulating layer 900; the gate insulating layer 300provided over the oxide semiconductor layer 410, the contact electrode510, and the contact electrode 520; and the gate electrode 200 providedover the gate insulating layer 300.

Note that the base insulating layer 900 is provided over the substrate100.

The insulating layer 600 is provided to cover the transistor.

A portion surrounded by the dashed line 8000 serves as a back channel.

Here, the base insulating layer 900 can be formed using a materialsimilar to the material of the insulating layer 600 described inEmbodiment 1. The base insulating layer 900 and the insulating layer 600may be formed using either the same material or different materials.

In the top-gate top-contact transistor, a portion in contact with theback channel is not the insulating layer 600 but the base insulatinglayer 900.

That is, a film in which the quantity of substances containing ahydrogen element is small is preferably used for the base insulatinglayer 900.

Note that in the transistor in FIG. 4B, an off-set region having a widthof several micrometers is formed between a channel formation region (aregion where the gate electrode and the oxide semiconductor layeroverlap with each other) and each of contact regions (regions where thewirings and the oxide semiconductor layer are in contact with eachother).

The off-set region has an advantage of reducing off current of thetransistor, but it has a disadvantage of reducing also on current of thetransistor.

On the other hand, unlike in FIG. 4B, an off-set region is not presentin FIGS. 15A and 15B; thus, unlike the transistor in FIG. 4B, thetransistors in FIGS. 15A and 15B have an advantage of improving oncurrent.

As described above, the transistor may employ any structure.

In other words, the transistor may employ any structure as long as thetransistor includes at least the gate electrode, the oxide semiconductorlayer, and the gate insulating layer provided between the gate electrodeand the oxide semiconductor layer.

A dual-gate transistor may be employed in which a first gate electrode,a first gate insulating layer over the first gate electrode, an oxidesemiconductor layer over the first gate insulating layer, a second gateinsulating layer over the oxide semiconductor layer, and a second gateelectrode over the second gate insulating layer are provided.

Therefore, the structure of the transistor is not limited to any of thestructures described in Embodiment 1 and 2.

The contents of this embodiment or part thereof can be implemented incombination with any of the other embodiments and examples.

Embodiment 3

An example of a method for manufacturing the semiconductor device inFIG. 4A will be described.

Note that materials, conditions of heat treatment, and the like are thesame as those in another embodiment.

The gate electrode 200 is formed over the substrate 100 having aninsulating surface. Then, the gate insulating layer 300 is formed overthe gate electrode 200, and the contact electrode 510 and the contactelectrode 520 are formed over the gate insulating layer 300 (FIG. 5A).

Next, the oxide semiconductor layer 400 is formed over the gateinsulating layer 300, the contact electrode 510, and the contactelectrode 520 (FIG. 5B).

Next, the oxide semiconductor layer is subjected to first heattreatment.

The conditions of the first heat treatment are the same as those inEmbodiment 1.

The first heat treatment may be performed after the oxide semiconductorlayer 400 is etched into an island shape to form the oxide semiconductorlayer 410.

However, after the oxide semiconductor layer 400 is etched into anisland shape to form the oxide semiconductor layer 410, the contactelectrodes are exposed.

When the first heat treatment is performed in the state where thecontact electrodes are exposed, surfaces of the contact electrodes areoxidized and the conductivity of the surfaces are reduced.

Therefore, the first heat treatment is preferably performed in the statewhere the contact electrodes are covered with the oxide semiconductorlayer 400.

Next, the oxide semiconductor layer 400 is etched into an island shapeto form the oxide semiconductor layer 410, and the insulating layer 600is formed to cover a transistor (FIG. 5C).

Note that a portion surrounded by the dashed line 8000 serves as a backchannel.

A structure may be employed in which, after the step of FIG. 5C, contactholes are formed in the insulating layer 600 and the wiring 810, thewiring 820, and the like are formed over the insulating layer 600 (FIG.6A).

Alternatively, a structure may be employed in which, after the step ofFIG. 5C, a contact hole is formed in the insulating layer 600 and apixel electrode 910 is formed over the insulating layer 600 (FIG. 6B).

After the wirings are formed over the insulating layer 600 asillustrated in FIG. 6A, an insulating layer, a wiring, a transistor, adisplay element, an antenna, or the like may be further formed over thewirings.

After the pixel electrode 910 is formed as illustrated in FIG. 6B, adisplay element (e.g., an EL element or a liquid crystal element) isformed, whereby a display device can be formed.

After the step of FIG. 6A or FIG. 6B, second heat treatment ispreferably performed.

The second heat treatment may be performed between the step of FIG. 5Cand the step of FIG. 6A or FIG. 6B.

The conditions of the second heat treatment are the same as those inEmbodiment 1.

The contents of this embodiment or part thereof can be implemented incombination with any of the other embodiments and examples.

Embodiment 4

An example of a method for manufacturing the semiconductor device inFIG. 4C will be described.

Note that materials, conditions of heat treatment, and the like are thesame as those in another embodiment.

First, the structure of FIG. 1B is formed as in Embodiment 1.

Note that first heat treatment is also performed as in Embodiment 1.

Next, the channel protective layer 700 (island shape) is formed, and theconductive layer 500 is formed to cover the channel protective layer 700(FIG. 7A).

Then, the conductive layer 500 is etched to form the contact electrode510 and the contact electrode 520 (FIG. 7B).

Note that a portion surrounded by the dashed line 8000 serves as a backchannel.

Because of the presence of the channel protective layer 700, the backchannel is not etched during the formation of the contact electrodes;therefore, damage to the back channel can be reduced.

Here, the channel protective layer 700 can be formed using a materialsimilar to the material of the insulating layer 600 described inEmbodiment 1. The channel protective layer 700 and the insulating layer600 may be formed using either the same material or different materials.

In the channel-stop transistor, a portion in contact with the backchannel is not the insulating layer 600 but the channel protective layer700.

That is, a film in which the quantity of substances containing ahydrogen element is small is preferably used for the channel protectivelayer 700.

The channel protective layer 700 is preferably formed using aninsulating layer containing excessive oxygen by a method similar to themethod described in Embodiment 1.

Next, the insulating layer 600 is formed to cover the transistor (FIG.7C).

A structure may be employed in which, after the step of FIG. 7C, contactholes are formed in the insulating layer 600 and the wiring 810, thewiring 820, and the like are formed over the insulating layer 600 (FIG.8A).

Alternatively, a structure may be employed in which, after the step ofFIG. 7C, a contact hole is formed in the insulating layer 600 and thepixel electrode 910 is formed over the insulating layer 600 (FIG. 8B).

After the wirings are formed over the insulating layer 600 asillustrated in FIG. 8A, an insulating layer, a wiring, a transistor, adisplay element, an antenna, or the like may be further formed over thewirings.

After the pixel electrode 910 is formed as illustrated in FIG. 8B, adisplay element (e.g., an EL element or a liquid crystal element) isformed, whereby a display device can be formed.

After the step of FIG. 8A or FIG. 8B, second heat treatment ispreferably performed.

The second heat treatment may be performed between the step of FIG. 7Cand the step of FIG. 8A or FIG. 8B.

The conditions of the second heat treatment are the same as those inEmbodiment 1.

The contents of this embodiment or part thereof can be implemented incombination with any of the other embodiments and examples.

Embodiment 5

As a semiconductor device, there are various integrated circuits.

For example, display devices (such as liquid crystal display devices andelectroluminescent display devices (light-emitting devices)),semiconductor devices for performing wireless communication throughantennas (such as RFID tags, wireless tags, IC chips, wireless chips,noncontact signal processing devices, and semiconductor integratedcircuit chips), and the like are given, but the integrated circuit isnot limited to these examples.

The contents of this embodiment or part thereof can be implemented incombination with any of the other embodiments and examples.

Example 1

An influence of the nitrogen concentration in an oxide semiconductor wasexamined.

First, an oxide semiconductor layer was formed over a glass substrate.

Then, the case where heat treatment was not performed after theformation of the oxide semiconductor layer and the case where heattreatment was performed after the formation of the oxide semiconductorlayer were compared.

They were compared by secondary ion mass spectrometry (SIMS).

Note that the heat treatment was performed at 350° C. for 1 hour.

The heating atmosphere was set to an air atmosphere or a nitrogenatmosphere.

Here, the oxide semiconductor layer was formed by sputtering using anoxide semiconductor target in which the atom ratio of In to Ga and Znwas 1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at a molar ratio)).

Note that before the formation of the oxide semiconductor layer, thereduction of leakage from a deposition chamber, the reduction ofnitrogen on the inner wall of the deposition chamber, deposition onto adummy substrate, or the like was sufficiently performed, so thatincorporation of nitrogen into the oxide semiconductor layer wasthoroughly prevented.

(Sample 1)

In Sample 1, the flow rate of a sputtering gas was set as follows:Ar/N₂=40/0 sccm (the proportion of N₂ was 0%). Note that three Samples 1were prepared.

The measurement results of SIMS of Sample 1 are shown in FIGS. 10A and10B.

(Sample 2)

In Sample 2, the flow rate of a sputtering gas was set as follows:Ar/N₂=35/5 sccm (the proportion of N₂ was 12.5%). Note that threeSamples 2 were prepared.

The measurement results of SIMS of Sample 2 are shown in FIGS. 11A and11B.

(Sample 3)

In Sample 3, the flow rate of a sputtering gas was set as follows:Ar/N₂=0/40 sccm (the proportion of N₂ was 100%). Note that three Samples3 were prepared.

The measurement results of SIMS of Sample 3 are shown in FIGS. 12A and12B.

(Consideration)

In each of FIGS. 10A and 10B, FIGS. 11A and 11B, and FIGS. 12A and 12B,a dotted line 3001 denotes the sample on which heat treatment was notperformed (such a sample is referred to as an “as-deposited sample”), athick solid line 3002 denotes the sample which was heated in a nitrogenatmosphere (such a sample is referred to as an “N₂-baked sample”), and athin solid line 3003 denotes the sample which was heated in an airatmosphere (such a sample is referred to as an “air-baked sample”).

FIG. 10A, FIG. 11A, and FIG. 12A show the hydrogen concentration, andFIG. 10B, FIG. 11B, and FIG. 12B show the nitrogen concentration.

In FIGS. 10A and 10B, FIGS. 11A and 11B, and FIGS. 12A and 12B, thevertical axis represents the concentration, and the horizontal axisrepresents the depth from a surface of the oxide semiconductor layer(the film thickness).

Note that the measurement results of secondary ion mass spectrometry(SIMS) have an effective range.

In the case of this example, in the vicinity of the surface of the oxidesemiconductor layer (at a depth of approximately 0 nm to 30 nm in FIGS.10A and 10B, FIGS. 11A and 11B, and FIGS. 12A and 12B) and in thevicinity of the interface between the oxide semiconductor layer and theglass substrate (at a depth of approximately 80 nm to 100 nm in FIGS.10A and 10B, FIGS. 11A and 11B, and FIGS. 12A and 12B), accurate valuesare difficult to evaluate.

Therefore, in the case of this example, the effective range of themeasurement results of SIMS was set to the range of 30 nm to 80 nm indepth.

Here, the average values of the hydrogen concentration in the effectiverange of the dotted line 3001 (the as-deposited sample) in FIG. 10A, thedotted line 3001 (the as-deposited sample) in FIG. 11A, and the dottedline 3001 (the as-deposited sample) in FIG. 12A are compared. It isfound that the average value in FIG. 11A is larger than that in FIG.10A, and the average value in FIG. 12A is larger than that in FIG. 11A.

Further, the average values of the nitrogen concentrations in theeffective range of the dotted line 3001 (the as-deposited sample) inFIG. 10B, the dotted line 3001 (the as-deposited sample) in FIG. 11B,and the dotted line 3001 (the as-deposited sample) in FIG. 12B arecompared. It is found that the average value in FIG. 11B is larger thanthat in FIG. 10B, and the average value in FIG. 12B is larger than thatin FIG. 11B.

That is, in the as-deposited samples, the higher the nitrogenconcentration in the oxide semiconductor layer is, the higher thehydrogen concentration in the oxide semiconductor layer becomes.

Therefore, the higher the nitrogen concentration in the oxidesemiconductor layer is, the more hydrogen is likely to enter the oxidesemiconductor layer.

With reference to FIG. 10A, the hydrogen concentration is decreasedthrough the heat treatment.

On the other hand, with reference to FIG. 11A and FIG. 12A, the hydrogenconcentration is increased through the heat treatment.

Note that in FIG. 12A, in the range of 30 nm to 60 nm, there is littledifference in the hydrogen concentration among the dotted line 3001 (theas-deposited sample), the thick solid line 3002 (the N₂-baked sample),and the thin solid line 3003 (the air-baked sample).

However, in the range of 30 nm to 80 nm which is the effective range,the average value of the hydrogen concentration of the thick solid line3002 (the N₂-baked sample) and that of the thin solid line 3003 (theair-baked sample) are larger than that of the dotted line 3001 (theas-deposited sample).

Thus, it is clear that, in the case where the total amounts of hydrogenin the oxide semiconductor layers are compared, the amount of hydrogenshown by the thick solid line 3002 (the N₂-baked sample) and that shownby the thin solid line 3003 (the air-baked sample) are larger than thatshown by the dotted line 3001 (the as-deposited sample).

It seem that the reason why the thick solid line 3002 (the N₂-bakedsample) and the thin solid line 3003 (the air-baked sample) have V-likeshapes in FIG. 12A is that hydrogen is injected from the surface of thefilm and the glass substrate.

Here, in Sample 1, the maximum value of the nitrogen concentration inthe effective range of the measurement results of SIMS was 1×10²⁰atoms/cm³ or less (less than 1×10²⁰ atoms/cm³) (FIG. 10B).

Therefore, in Sample 1, the average value of the nitrogen concentrationin the effective range of the measurement results of SIMS was also1×10²⁰ atoms/cm³ or less (less than 1×10²⁰ atoms/cm³) (the average valueis not larger than the maximum value).

Note that, as for the as-deposited Sample 1 (FIGS. 10A and 10B), themaximum value of the nitrogen concentration in the effective range ofthe measurement results of SIMS was 9.3×10¹⁹ atoms/cm³, the minimumvalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 1.9×10¹⁹ atoms/cm³, and the averagevalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 6.1×10¹⁹ atoms/cm³.

Further, as for the as-deposited Sample 1 (FIGS. 10A and 10B), themaximum value of the hydrogen concentration in the effective range ofthe measurement results of SIMS was 6.9×10¹⁹ atoms/cm³, the minimumvalue of the hydrogen concentration in the effective range of themeasurement results of SIMS was 4.5×10¹⁹ atoms/cm³, and the averagevalue of the hydrogen concentration in the effective range of themeasurement results of SIMS was 5.6×10¹⁹ atoms/cm³.

Note that, as for the N₂-baked Sample 1 (FIGS. 10A and 10B), the maximumvalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 9.7×10¹⁹ atoms/cm³, the minimum value ofthe nitrogen concentration in the effective range of the measurementresults of SIMS was 3.0×10¹⁹ atoms/cm³, and the average value of thenitrogen concentration in the effective range of the measurement resultsof SIMS was 6.0×10¹⁹ atoms/cm³.

Further, as for the N₂-baked Sample 1 (FIGS. 10A and 10B), the maximumvalue of the hydrogen concentration in the effective range of themeasurement results of SIMS was 2.3×10¹⁹ atoms/cm³, the minimum value ofthe hydrogen concentration in the effective range of the measurementresults of SIMS was 6.4×10¹⁸ atoms/cm³, and the average value of thehydrogen concentration in the effective range of the measurement resultsof SIMS was 1.2×10¹⁹ atoms/cm³.

Note that, as for the air-baked Sample 1 (FIGS. 10A and 10B), themaximum value of the nitrogen concentration in the effective range ofthe measurement results of SIMS was 3.1×10¹⁹ atoms/cm³, the minimumvalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 4.4×10¹⁸ atoms/cm³, and the averagevalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 1.8×10¹⁹ atoms/cm³.

Further, as for the air-baked Sample 1 (FIGS. 10A and 10B), the maximumvalue of the hydrogen concentration in the effective range of themeasurement results of SIMS was 6.7×10¹⁸ atoms/cm³, the minimum value ofthe hydrogen concentration in the effective range of the measurementresults of SIMS was 2.0×10¹⁸ atoms/cm³, and the average value of thehydrogen concentration in the effective range of the measurement resultsof SIMS was 3.8×10¹⁸ atoms/cm³.

On the other hand, in Sample 2 and Sample 3, the minimum value of thenitrogen concentration in the effective range of the measurement resultsof SIMS was 1×10²² atoms/cm³ or more (FIG. 11B and FIG. 12B).

Therefore, in Sample 2 and Sample 3, the average value of the nitrogenconcentration in the effective range of the measurement results of SIMSwas also 1×10²² atoms/cm³ or more (the average value is not smaller thanthe maximum value).

Note that, as for the as-deposited Sample 2 (FIGS. 11A and 11B), themaximum value of the nitrogen concentration in the effective range ofthe measurement results of SIMS was 1.6×10²² atoms/cm³, the minimumvalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 1.5×10²² atoms/cm³, and the averagevalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 1.5×10²² atoms/cm³.

Further, as for the as-deposited Sample 2 (FIGS. 11A and 11B), themaximum value of the hydrogen concentration in the effective range ofthe measurement results of SIMS was 1.0×10²⁰ atoms/cm³, the minimumvalue of the hydrogen concentration in the effective range of themeasurement results of SIMS was 6.3×10¹⁹ atoms/cm³, and the averagevalue of the hydrogen concentration in the effective range of themeasurement results of SIMS was 7.8×10¹⁹ atoms/cm³.

Note that, as for the N₂-baked Sample 2 (FIGS. 11A and 11B), the maximumvalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 1.5×10²² atoms/cm³, the minimum value ofthe nitrogen concentration in the effective range of the measurementresults of SIMS was 1.4×10²² atoms/cm³, and the average value of thenitrogen concentration in the effective range of the measurement resultsof SIMS was 1.5×10²² atoms/cm³.

Further, as for the N₂-baked Sample 2 (FIGS. 11A and 11B), the maximumvalue of the hydrogen concentration in the effective range of themeasurement results of SIMS was 1.2×10²¹ atoms/cm³, the minimum value ofthe hydrogen concentration in the effective range of the measurementresults of SIMS was 6.5×10²⁰ atoms/cm³, and the average value of thehydrogen concentration in the effective range of the measurement resultsof SIMS was 7.7×10²⁰ atoms/cm³.

Note that, as for the air-baked Sample 2 (FIGS. 11A and 11B), themaximum value of the nitrogen concentration in the effective range ofthe measurement results of SIMS was 1.6×10²² atoms/cm³, the minimumvalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 1.5×10²² atoms/cm³, and the averagevalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 1.6×10²² atoms/cm³.

Further, as for the air-baked Sample 2 (FIGS. 11A and 11B), the maximumvalue of the hydrogen concentration in the effective range of themeasurement results of SIMS was 6.9×10²⁰ atoms/cm³, the minimum value ofthe hydrogen concentration in the effective range of the measurementresults of SIMS was 4.4×10²⁰ atoms/cm³, and the average value of thehydrogen concentration in the effective range of the measurement resultsof SIMS was 5.3×10²⁰ atoms/cm³.

Note that, as for the as-deposited Sample 3 (FIGS. 12A and 12B), themaximum value of the nitrogen concentration in the effective range ofthe measurement results of SIMS was 3.4×10²² atoms/cm³, the minimumvalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 3.1×10²² atoms/cm³, and the averagevalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 3.3×10²² atoms/cm³.

Further, as for the as-deposited Sample 3 (FIGS. 12A and 12B), themaximum value of the hydrogen concentration in the effective range ofthe measurement results of SIMS was 1.9×10²⁰ atoms/cm³, the minimumvalue of the hydrogen concentration in the effective range of themeasurement results of SIMS was 1.1×10²⁰ atoms/cm³, and the averagevalue of the hydrogen concentration in the effective range of themeasurement results of SIMS was 1.4×10²⁰ atoms/cm³.

Note that, as for the N₂-baked Sample 3 (FIGS. 12A and 12B), the maximumvalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 3.3×10²² atoms/cm³, the minimum value ofthe nitrogen concentration in the effective range of the measurementresults of SIMS was 3.2×10²² atoms/cm³, and the average value of thenitrogen concentration in the effective range of the measurement resultsof SIMS was 3.2×10²² atoms/cm³.

Further, as for the N₂-baked Sample 3 (FIGS. 12A and 12B), the maximumvalue of the hydrogen concentration in the effective range of themeasurement results of SIMS was 8.6×10²⁰ atoms/cm³, the minimum value ofthe hydrogen concentration in the effective range of the measurementresults of SIMS was 1.4×10²⁰ atoms/cm³, and the average value of thehydrogen concentration in the effective range of the measurement resultsof SIMS was 2.7×10²⁰ atoms/cm³.

Note that, as for the air-baked Sample 3 (FIGS. 12A and 12B), themaximum value of the nitrogen concentration in the effective range ofthe measurement results of SIMS was 3.5×10²² atoms/cm³, the minimumvalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 3.3×10²² atoms/cm³, and the averagevalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 3.4×10²² atoms/cm³.

Further, as for the air-baked Sample 3 (FIGS. 12A and 12B), the maximumvalue of the hydrogen concentration in the effective range of themeasurement results of SIMS was 1.0×10²¹ atoms/cm³, the minimum value ofthe hydrogen concentration in the effective range of the measurementresults of SIMS was 7.6×10¹⁹ atoms/cm³, and the average value of thehydrogen concentration in the effective range of the measurement resultsof SIMS was 2.8×10²⁰ atoms/cm³.

As described above, it is found that hydrogen is easily absorbed by theoxide semiconductor layer containing nitrogen.

In other words, an oxide semiconductor layer whose average value of thenitrogen concentration in the effective range of the measurement resultsof SIMS is 1×10²⁰ atoms/cm³ or less (less than 1×10²⁰ atoms/cm³) is adense layer which is difficult for hydrogen to enter.

In Samples 1 to 3, the nitrogen concentration in the oxide semiconductorlayer was not increased even through the heat treatment. In Samples 1,in the case where the air-baking was performed, the nitrogenconcentration in the oxide semiconductor layer was decreased.

Here, it is assumed that the reason why nitrogen is absorbed by theoxide semiconductor layer is that part of an oxygen bond in the oxidesemiconductor layer is cut off during formation of the oxidesemiconductor layer and a nitrogen bond is formed in a position wherethe oxide bond has been cut off.

Therefore, it is hypothesized that the concentration of nitrogen whichis to be contained in the oxide semiconductor layer can be decreased inthe case where an oxygen bond is formed in a position where anotheroxide bond has been cut off. Note that when these assumption andhypothesis are valid, oxide semiconductors other than anIn—Ga—Zn—O-based oxide semiconductor are also influenced by nitrogen.

Example 2

In order to demonstrate the validity of the assumption and hypothesis inExample 1, an oxide semiconductor layer was formed using oxygen as asputtering gas.

When oxygen is supplied, an oxygen bond should be more likely to beformed in a position where another oxygen bond has been cut off.

The oxide semiconductor layer was formed by sputtering using an oxidesemiconductor target in which the atom ratio of In to Ga and Zn was1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at a molar ratio))

Note that before the formation of the oxide semiconductor layer, thereduction of leakage from a deposition chamber, the reduction ofnitrogen on the inner wall of the deposition chamber, deposition onto adummy substrate, or the like was sufficiently performed, so thatincorporation of nitrogen into the oxide semiconductor layer wasthoroughly prevented.

(Sample 4)

In Sample 4, the flow rate of a sputtering gas was set as follows:Ar/O₂=30/15 sccm (the proportion of O₂ was 33.3%).

The measurement results of SIMS of Sample 4 are shown in FIG. 13A.

(Sample 5)

In Sample 5 (the as-deposited sample), the flow rate of a sputtering gaswas set as follows: Ar/O₂=0/40 sccm (the proportion of O₂ was 100%).

The measurement results of SIMS of Sample 5 are shown in FIG. 13B.

(Consideration)

The effective range of the measurement results of SIMS is the same asthat in Example 1.

Here, in Sample 4 (FIG. 13A), the maximum value of the nitrogenconcentration in the effective range of the measurement results of SIMSwas 5×10¹⁹ atoms/cm³ or less (less than 5×10¹⁹ atoms/cm³).

Therefore, in Sample 4 (FIG. 13A), the average value of the nitrogenconcentration in the effective range of the measurement results of SIMSwas also 5×10¹⁹ atoms/cm³ or less (less than 5×10¹⁹ atoms/cm³) (theaverage value is not larger than the maximum value).

In Sample 4 (FIG. 13A), the average value of the nitrogen concentrationin the effective range of the measurement results of SIMS wascalculated. The average value was 1×10¹⁹ atoms/cm³ or less (less than1×10¹⁹ atoms/cm³).

In Sample 5 (FIG. 13B), the maximum value of the nitrogen concentrationin the effective range of the measurement results of SIMS was 2×10¹⁹atoms/cm³ or less (less than 2×10¹⁹ atoms/cm³).

Therefore, in Sample 5 (FIG. 13B), the average value of the nitrogenconcentration in the effective range of the measurement results of SIMSwas also 1×10¹⁹ atoms/cm³ or less (less than 1×10¹⁹ atoms/cm³).

Sample 4 (FIG. 13A) and Sample 5 (FIG. 13B) have a lower nitrogenconcentration than Sample 1 (FIG. 10B) to which oxygen was not added.

Note that, as for the as-deposited Sample 4 (FIG. 13A), the maximumvalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 1.6×10¹⁹ atoms/cm³, the minimum value ofthe nitrogen concentration in the effective range of the measurementresults of SIMS was 2.2×10¹⁸ atoms/cm³, and the average value of thenitrogen concentration in the effective range of the measurement resultsof SIMS was 4.6×10¹⁸ atoms/cm³.

Note that, as for the as-deposited Sample 5 (FIG. 13B), the maximumvalue of the nitrogen concentration in the effective range of themeasurement results of SIMS was 1.6×10¹⁹ atoms/cm³, the minimum value ofthe nitrogen concentration in the effective range of the measurementresults of SIMS was 2.3×10¹⁸ atoms/cm³, and the average value of thenitrogen concentration in the effective range of the measurement resultsof SIMS was 7.7×10¹⁸ atoms/cm³.

Therefore, the validity of the assumption and hypothesis in Example 1was demonstrated.

Example 3

The difference in electrical characteristics of transistors due to thedifference in materials of the insulating layer 600 was examined.

(Common Condition)

The channel-etched transistor in FIG. 2B was manufactured.

For the oxide semiconductor layer 410, a sample formed in the followingmanner was used: an oxide semiconductor layer was formed by sputteringusing an oxide semiconductor target in which the atom ratio of In to Gaand Zn was 1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at a molar ratio)), and theoxide semiconductor layer was etched into an island shape.

After the step of FIG. 1B, first heat treatment was performed at 350° C.for 1 hour in an air atmosphere.

After the step of FIG. 2B, a contact hole was formed in the insulatinglayer 600, and a wiring was formed over the insulating layer 600.

After the wiring was formed, second heat treatment was performed at 350°C. for 1 hour in an air atmosphere.

(Sample 6)

The insulating layer 600 was formed by a sputtering method using asilicon oxide target.

The insulating layer 600 was formed under the following conditions. Thesubstrate temperature was set to 100° C. The flow rate of a sputteringgas was set as follows: Ar/O₂=40/10 sccm.

The RF power source was used, and the power of the RF power source wasset to 1.5 kW.

The deposition pressure was set to 0.4 Pa.

(Sample 7)

The insulating layer 600 was formed by a sputtering method using asilicon oxide target.

The insulating layer 600 was formed under the following conditions. Thesubstrate temperature was set to 250° C. The flow rate of a sputteringgas was set as follows: Ar/O₂=40/10 sccm.

The RF power source was used, and the power of the RF power source wasset to 1.5 kW.

The deposition pressure was set to 0.4 Pa.

(Sample 8)

The insulating layer 600 was formed by a sputtering method using asilicon oxide target.

The insulating layer 600 was formed under the following conditions. Thesubstrate temperature was set to 100° C. The flow rate of a sputteringgas was set as follows: Ar/H₂=46/4 sccm.

The RF power source was used, and the power of the RF power source wasset to 1.5 kW.

The deposition pressure was set to 0.4 Pa.

(Sample 9)

The insulating layer 600 was formed by a plasma CVD method.

The insulating layer 600 was formed under the following conditions. Thesubstrate temperature was set to 200° C. The flow rate of depositiongases was set as follows: SiH₄/N₂O=25/1000 sccm.

(Sample 10)

The insulating layer 600 was formed by a plasma CVD method.

The insulating layer 600 was formed under the following conditions. Thesubstrate temperature was set to 325° C. The flow rate of depositiongases was set as follows: SiH₄/N₂O=27/1000 sccm.

(Sample 11)

The insulating layer 600 was formed using a siloxane film.

(Sample 12)

The insulating layer 600 was formed using an acrylic film.

(Sample 13)

The insulating layer 600 was formed using a polyimide film.

(Consideration)

The threshold voltage Vth of Samples 8 to 13 shifted in a negativedirection as compared with that of Samples 6 and 7.

Here, the insulating layer 600 formed with the use of a substance whichis intentionally made to contain a hydrogen element contains a hydrogenelement.

Consequently, the insulating layer 600 containing hydrogen is in contactwith a back channel.

The insulating layer 600 formed by a plasma CVD method contains hydrogenand nitrogen.

Consequently, the insulating layer 600 containing hydrogen is in contactwith the back channel.

The siloxane film, the acrylic film, and the polyimide film easilyabsorb moisture and easily release moisture.

Consequently, the insulating layer 600 which releases moisture is incontact with the back channel.

Here, with the use of computational science (it is also calledsimulation although there is a difference between computational scienceand simulation in a strict sense), calculation was performed to examinebehavior in the case where a donor was present in the back channel. Thecalculation showed that, in the case where donor was present in the backchannel, the threshold voltage shifted in a negative direction.

Therefore, it was found that, when the insulating layer 600 formed underthe condition where hydrogen was not used as a sputtering gas was used,the threshold voltage was prevented from shifting in a negativedirection.

In addition, it was found that oxygen was supplied to the back channelthrough the second heat treatment and oxygen deficiency in the backchannel was reduced in the case where the insulating layer 600containing excessive oxygen which was formed using oxygen as asputtering gas and using a silicon oxide target was used.

Note that in this example, the second heat treatment was performed at350° C. for 1 hour.

In the case where the insulating layer 600 was formed using aninsulating layer containing excessive oxygen, Sample A on which thesecond heat treatment has not been performed and Sample B on which thesecond heat treatment has been performed at 250° C. for 1 hour wereprepared. Then, Sample A and Sample B were compared. The results showedthat Sample B on which the second heat treatment has been performed at250° C. for 1 hour had more excellent electrical characteristics of thetransistor than Sample A.

Further, a transistor including the insulating layer 600 formed usingonly oxygen as a sputtering gas and using a silicon target wasseparately prepared.

The case where the insulating layer 600 was formed using only oxygen asa sputtering gas and using a silicon target at a substrate temperatureof 100° C. and the case where the insulating layer 600 was formed usingonly oxygen as a sputtering gas and using a silicon target at asubstrate temperature of 200° C. were compared. The results showed thatthe case where the substrate temperature was set to 200° C. was betterthan the case where the substrate temperature was set to 100° C., in theBT test.

The reason why the case where the substrate temperature was set to 200°C. was better in the BT test was found to be that, by increase in thesubstrate temperature during the formation of the insulating layer 600,moisture on a surface of the back channel was removed.

Therefore, the substrate temperature during the formation of theinsulating layer 600 is preferably 200° C. or higher (the upper limit isnot limited, but can be set to 300° C. or lower, 400° C. or lower, 500°C. or lower, 600° C. or lower, or 700° C. or lower).

Example 4

The channel-etched transistor in FIG. 2A (the In—Ga—Zn—O-basedtransistor) was manufactured. Note that the insulating layer 600 was notformed.

Then, a substrate over which the transistor has been formed was immersedin water.

The characteristics obtained before the substrate over which thetransistor has been formed was immersed in water and the characteristicsobtained just after the substrate over which the transistor has beenformed was immersed in water were compared. The results showed that,just after the substrate over which the transistor has been formed wasimmersed in water, the threshold voltage shifted in a negative directionand off current was increased as compared with those before thesubstrate over which the transistor has been formed was immersed inwater.

Next, the substrate which has been immersed in water was heated at 120°C. for 3 minutes in an air atmosphere to be dried, whereby thecharacteristics of the transistor were slightly recovered.

Next, the substrate which has been dried for 3 minutes was heated at120° C. for 10 minutes (13 minutes in total) in an air atmosphere,whereby the characteristics of the transistor were further recovered.

Next, the substrate which has been dried for 13 minutes was heated at120° C. for 40 minutes (53 minutes in total) in an air atmosphere,whereby the characteristics of the transistor became almost the sameones obtained before the substrate over which the transistor has beenformed was immersed in water.

Thus, it is found that when moisture is attached to the surface of theback channel, the characteristics of the transistor are influenced.

A small amount of moisture such as the one which is attached to thesurface of the back channel can be removed by heat treatment at 120° C.for 53 minutes or more.

Note that only the evaporation of moisture needs to be achieved, and itis therefore clear that increasing the temperature shortens the dryingtime.

Example 5

The effect of heat treatment performed on an oxide semiconductor layerwas examined using thermal desorption spectroscopy (TDS).

Thermal desorption spectroscopy (TDS) is a method for analyzing a gasemitted when the temperature of a sample is increased.

In this example, emission of water vapor was examined.

(Sample 14)

As Sample 14, a glass substrate was prepared.

(Sample 15)

As Sample 15, a sample formed in the following manner was prepared: anoxide semiconductor layer was formed over a glass substrate bysputtering using an oxide semiconductor target in which the atom ratioof In to Ga and Zn was 1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at a molarratio)).

Sample 15 has not been subjected to heat treatment (the as-depositedsample).

(Sample 16)

As Sample 16, the same sample as Sample 15 was prepared.

(Sample 17)

As Sample 17, a sample formed in the following manner was prepared: anoxide semiconductor layer was formed over a glass substrate bysputtering using an oxide semiconductor target in which the atom ratioof In to Ga and Zn was 1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at a molarratio)).

After the oxide semiconductor layer was formed, heat treatment wasperformed at 250° C. for 1 hour in a nitrogen atmosphere.

(Sample 18)

As Sample 18, a sample formed in the following manner was prepared: anoxide semiconductor layer was formed over a glass substrate bysputtering using an oxide semiconductor target in which the atom ratioof In to Ga and Zn was 1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at a molarratio)).

After the oxide semiconductor layer was formed, heat treatment wasperformed at 350° C. for 1 hour in a nitrogen atmosphere.

(Sample 19)

As Sample 19, a sample formed in the following manner was prepared: anoxide semiconductor layer was formed over a glass substrate bysputtering using an oxide semiconductor target in which the atom ratioof In to Ga and Zn was 1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at a molarratio)).

After the oxide semiconductor layer was formed, heat treatment wasperformed at 450° C. for 1 hour in a nitrogen atmosphere.

(Consideration)

The results of the measurement using thermal desorption spectroscopy ofSample 14 and Sample 15 are shown in FIG. 14A.

Note that in FIG. 14A, a graph 1414 corresponds to a graph of Sample 14(only the glass substrate), and a graph 1415 corresponds to a graph ofSample 15 (the glass substrate and the oxide semiconductor layer (theas-deposited sample)).

In FIG. 14A, there are a peak 1401 of moisture at around 50° C., a peak1402 of moisture at around 100° C., and a peak 1403 of moisture ataround 300° C.

As for Sample 14 including only the glass substrate, there are only thepeak 1401 of moisture at around 50° C. and the peak 1402 of moisture ataround 100° C., and there is not the peak 1403 of moisture at around300° C.

On the other hand, as for Sample 15 including the oxide semiconductorlayer, there are the peak 1401 of moisture at around 50° C., the peak1402 of moisture at around 100° C., and the peak 1403 of moisture ataround 300° C.

Therefore, the peak 1403 of moisture at around 300° C. is a peak ofmoisture which is characteristic of the oxide semiconductor layer.

Thus, in the case where the peak 1403 of moisture at around 300° C. isdetected, moisture should be contained in the oxide semiconductor layer.

The results of comparing Samples 16 to 19 are shown in FIG. 14B.

Note that in FIG. 14B, a graph 1416 corresponds to a graph of Sample 16(the as-deposited sample), a graph 1417 corresponds to a graph of Sample17 (the sample heated at 250° C.), a graph 1418 corresponds to a graphof Sample 18 (the sample heated at 350° C.), and a graph 1419corresponds to a graph of Sample 19 (the sample heated at 450° C.).

As for Sample 17 (250° C.) and Sample 18 (350° C.), the peak 1403 ofmoisture at around 300° C. was detected.

The number of the peaks 1403 of moisture at around 300° C. of Sample 17(250° C.) and Sample 18 (350° C.) is smaller than that of Sample 16 (theas-deposited sample).

Thus, it was found that a certain amount of moisture was reduced throughthe heat treatment.

As for Sample 19 (450° C.), there is not the peak 1403 of moisture ataround 300° C.

Therefore, it can be said that, in the case where heat treatment isperformed at 450° C. for 1 hour or more, moisture is thoroughlyeliminated.

(Sample 20)

As Sample 20, a sample formed in the following manner was prepared: anoxide semiconductor layer was formed over a glass substrate bysputtering using an oxide semiconductor target in which the atom ratioof In to Ga and Zn was 1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at a molarratio)).

Then, Sample 20 was subjected to heat treatment at 650° C. for 3 minutesin a nitrogen atmosphere with the use of a gas RTA apparatus.

As for Sample 20 (650° C.), there was not the peak of moisture at around300° C.

Thus, it can be said that, in the case where heat treatment is performedat 650° C. for 3 minutes or more, moisture is thoroughly eliminated.

Samples formed in the following manner were prepared: an oxidesemiconductor layer was formed over a glass substrate by sputteringusing an oxide semiconductor target in which the atom ratio of In to Gaand Zn was 1:1:1 (In₂O₃:Ga₂O₃:ZnO=1:1:2 (at a molar ratio)). Experimentssimilar to the above (comparison between the as-deposited sample, thesample heated at 250° C., the sample heated at 350° C., and the sampleheated at 450° C.) were performed. The results showed that, as for onlythe sample on which the heat treatment was performed at 450° C. for 1hour, there was not the peak of moisture at around 300° C.

Samples formed in the following manner were prepared: an oxidesemiconductor layer was formed over a glass substrate by sputteringusing an oxide semiconductor target in which the atom ratio of In to Gaand Zn was 1:1:4 (In₂O₃:Ga₂O₃:ZnO=1:1:8 (at a molar ratio)). Experimentssimilar to the above (comparison between the as-deposited sample, thesample heated as 250° C., the sample heated at 350° C., and the sampleheated at 450° C.) were performed. The results showed that, as for onlythe sample on which the heat treatment was performed at 450° C. for 1hour, there was not the peak of moisture at around 300° C.

Therefore, it can be said that the oxide semiconductor layer which doesnot have a peak of moisture at around 300° C. in thermal desorptionspectroscopy (the number of peaks is 2×10⁻¹¹ or less at higher than orequal to 100° C. and lower than or equal to 400° C. (preferably, higherthan or equal to 250° C. and lower than or equal to 300° C.) is an oxidesemiconductor layer on which heat treatment has been performed at 450°C. or higher for 1 hour or more, or at 650° C. or higher for 3 minutesor more.

The absence of a peak of moisture at around 300° C. in thermaldesorption spectroscopy means the absence of a peak of moisture athigher than or equal to 100° C. and lower than or equal to 400° C. inthermal desorption spectroscopy.

The absence of a peak of moisture at around 300° C. in thermaldesorption spectroscopy means the absence of a peak of moisture due tothe oxide semiconductor layer.

When the transistors having the structure of FIG. 2A were manufacturedwith only the conditions of first heat treatment changed, thetransistors (250° C. and 350° C.) including an oxide semiconductor layerhaving a peak of moisture at around 300° C. in thermal desorptionspectroscopy had threshold voltage shifted in a negative direction andhad significant deterioration in the BT test compared with thetransistor (450° C.) including an oxide semiconductor layer which didnot have a peak of moisture at around 300° C. in thermal desorptionspectroscopy.

Note that as the oxide semiconductor layer, an oxide semiconductor layerformed by sputtering using an oxide semiconductor target in which theatom ratio of In to Ga and Zn was 1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at amolar ratio)) was used.

Note that before the formation of the oxide semiconductor layer, thereduction of leakage from a deposition chamber, the reduction ofnitrogen on the inner wall of the deposition chamber, deposition onto adummy substrate, or the like was sufficiently performed, so that aninfluence of nitrogen on the oxide semiconductor layer was thoroughlyprevented.

Thus, the electrical characteristics of a transistor were found to beinfluenced by moisture in an oxide semiconductor layer.

Example 6

Degasification (moisture) of the following samples was measured bythermal desorption spectroscopy (TDS).

(Sample 21)

As Sample 21, a sample formed in the following manner was prepared: anoxide semiconductor layer was formed over a glass substrate bysputtering using an oxide semiconductor target in which the atom ratioof In to Ga and Zn was 1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at a molarratio)), and a silicon oxide film was formed over the oxidesemiconductor layer by a sputtering method.

For the formation of the silicon oxide film, silicon oxide was used as atarget and the flow rate of gases was set as follows: Ar/O₂=40/10 sccm.

Sample 21 has not been subjected to heat treatment.

(Sample 22)

As Sample 22, a sample formed in the following manner was prepared: anoxide semiconductor layer was formed over a glass substrate bysputtering using an oxide semiconductor target in which the atom ratioof In to Ga and Zn was 1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at a molarratio)), and a silicon oxide film was formed over the oxidesemiconductor layer by a sputtering method.

For the formation of the silicon oxide film, silicon oxide was used as atarget and the flow rate of gases was set as follows: Ar/O₂=40/10 sccm(oxygen was used as a sputtering gas).

After that, heat treatment (in a furnace) was performed at 250° C. for 1hour in a nitrogen atmosphere.

(Sample 23)

As Sample 23, a sample formed in the following manner was prepared: anoxide semiconductor layer was formed over a glass substrate bysputtering using an oxide semiconductor target in which the atom ratioof In to Ga and Zn was 1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at a molarratio)), and a silicon oxide film was formed over the oxidesemiconductor layer by a sputtering method.

For the formation of the silicon oxide film, silicon oxide was used as atarget and the flow rate of gases was set as follows: Ar/O₂=40/10 sccm(oxygen was used as a sputtering gas).

After that, heat treatment (in a furnace) was performed at 350° C. for 1hour in a nitrogen atmosphere.

(Sample 24)

As Sample 24, a sample formed in the following manner was prepared: anoxide semiconductor layer was formed over a glass substrate bysputtering using an oxide semiconductor target in which the atom ratioof In to Ga and Zn was 1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at a molarratio)), and a silicon oxide film was formed over the oxidesemiconductor layer by a sputtering method.

For the formation of the silicon oxide film, silicon oxide was used as atarget and the flow rate of gases was set as follows: Ar/O₂=40/10 sccm(oxygen was used as a sputtering gas).

After that, heat treatment (in a furnace) was performed at 450° C. for 1hour in a nitrogen atmosphere.

(Consideration)

When the measurement was performed by thermal desorption spectroscopy(TDS), all the samples did not have a peak of moisture at around 300° C.

From the results of Example 5, moisture is not thoroughly removed byheat treatment at least at 350° C. or lower.

That is, Sample 23 and Sample 24 were supposed to have a peak ofmoisture at around 300° C.; however, a peak of moisture at around 300°C. was not detected.

The reason of this was found to be that the silicon oxide film formedusing oxygen as a sputtering gas (i.e., the silicon oxide filmcontaining excessive oxygen) blocked emission of moisture.

Therefore, it was found that the silicon oxide film formed using oxygenas a sputtering gas (i.e., the silicon oxide film containing excessiveoxygen) prevented diffusion of moisture.

In other words, it was found that the silicon oxide film containingexcessive oxygen had an effect of blocking moisture.

Conversely, in the case where a silicon oxide film containing excessiveoxygen is formed over an oxide semiconductor layer from which moisturehas been released by first heat treatment, incorporation of moisturefrom the outside can be prevented.

Example 7

From the results of Example 5, it was found that moisture was thoroughlyremoved by heat treatment at 450° C. for 1 hour or more.

Thus, the hydrogen concentration in a sample which has been heated at atemperature higher than 450° C. was examined by secondary ion massspectrometry (SIMS).

First, a plurality of samples in each of which an oxide semiconductorlayer was formed over a glass substrate by sputtering using an oxidesemiconductor target in which the atom ratio of In to Ga and Zn was1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at a molar ratio)) were prepared.

Further, a plurality of samples in each of which an oxide semiconductorlayer was formed over a glass substrate by sputtering using an oxidesemiconductor target in which the atom ratio of In to Ga and Zn was1:1:1 (In₂O₃:Ga₂O₃:ZnO=1:1:2 (at a molar ratio)) were prepared.

Note that before the formation of the oxide semiconductor layer, thereduction of leakage from a deposition chamber, the reduction ofnitrogen on the inner wall of the deposition chamber, deposition onto adummy substrate, or the like was sufficiently performed, so thatincorporation of nitrogen into the oxide semiconductor layer wasthoroughly prevented.

The flow rate of a sputtering gas was set as follows: Ar/O₂=30/15 sccm(the proportion of O₂ was 33.3%).

Then, a plurality of samples which have been subjected to heat treatmentat 450° C., 550° C., 600° C., and 650° C., respectively, were prepared.Note that the heating time for each sample was 1 hour.

Further, a plurality of samples which have been subjected to heattreatment in a nitrogen atmosphere, an oxygen atmosphere, and anatmosphere containing nitrogen and oxygen (dry air (i.e., an atmospherein which the ratio of nitrogen to oxygen is 4:1)), respectively, wereprepared.

The results of comparison are shown in Table 1.

TABLE 1 N2 O2 Dry Air 450° C. 1 hour [heat treatment] SputteringIn:Ga:Zn = 2 × 10¹⁹ 2 × 10¹⁹ 2 × 10¹⁹ apparatus A 1:1:0.5 (atomic ratio)atoms/cm³ atoms/cm³ atoms/cm³ (In2O3:Ga2O3:ZnO = 1:1:1 (molar ratio))In:Ga:Zn = 3 × 10¹⁹ 2 × 10¹⁹ 2 × 10¹⁹ 1:1:1 (atomic ratio) atoms/cm³atoms/cm³ atoms/cm³ (In2O3:Ga2O3:ZnO = 1:1:2 (molar ratio)) SputteringIn:Ga:Zn = 3 × 10¹⁹ 2 × 10¹⁹ 2 × 10¹⁹ apparatus B 1:1:0.5 (atomic ratio)atoms/cm³ atoms/cm³ atoms/cm³ (In2O3:Ga2O3:ZnO = 1:1:1 (molar ratio))In:Ga:Zn = 4 × 10¹⁹ 2 × 10¹⁹ 2 × 10¹⁹ 1:1:1 (atomic ratio) atoms/cm³atoms/cm³ atoms/cm³ (In2O3:Ga2O3:ZnO = 1:1:2 (molar ratio)) 550° C. 1hour [heat treatment] Sputtering In:Ga:Zn = 3 × 10¹⁸ 4 × 10¹⁸ 4 × 10¹⁸apparatus A 1:1:0.5 (atomic ratio) atoms/cm³ atoms/cm³ atoms/cm³(In2O3:Ga2O3:ZnO = 1:1:1 (molar ratio)) In:Ga:Zn = 3 × 10¹⁸ 4 × 10¹⁸ 4 ×10¹⁸ 1:1:1 (atomic ratio) atoms/cm³ atoms/cm³ atoms/cm³ (In2O3:Ga2O3:ZnO= 1:1:2 (molar ratio)) Sputtering In:Ga:Zn = 3 × 10¹⁸ 4 × 10¹⁸ 4 × 10¹⁸apparatus B 1:1:0.5 (atomic ratio) atoms/cm³ atoms/cm³ atoms/cm³(In2O3:Ga2O3:ZnO = 1:1:1 (molar ratio)) In:Ga:Zn = 3 × 10¹⁸ 4 × 10¹⁸ 4 ×10¹⁸ 1:1:1 (atomic ratio) atoms/cm³ atoms/cm³ atoms/cm³ (In2O3:Ga2O3:ZnO= 1:1:2 (molar ratio)) 600° C. 1 hour [heat treatment] SputteringIn:Ga:Zn = 6 × 10¹⁸ 4 × 10¹⁸ 4 × 10¹⁸ apparatus A 1:1:0.5 (atomic ratio)atoms/cm³ atoms/cm³ atoms/cm³ (In2O3:Ga2O3:ZnO = 1:1:1 (molar ratio))In:Ga:Zn = 4 × 10¹⁸ 3 × 10¹⁸ 3 × 10¹⁸ 1:1:1 (atomic ratio) atoms/cm³atoms/cm³ atoms/cm³ (In2O3:Ga2O3:ZnO = 1:1:2 (molar ratio)) SputteringIn:Ga:Zn = 4 × 10¹⁸ 5 × 10¹⁸ 4 × 10¹⁸ apparatus B 1:1:0.5 (atomic ratio)atoms/cm³ atoms/cm³ atoms/cm³ (In2O3:Ga2O3:ZnO = 1:1:1 (molar ratio))In:Ga:Zn = 4 × 10¹⁸ 4 × 10¹⁸ 4 × 10¹⁸ 1:1:1 (atomic ratio) atoms/cm³atoms/cm³ atoms/cm³ (In2O3:Ga2O3:ZnO = 1:1:2 (molar ratio)) 650° C. 1hour [heat treatment] Sputtering In:Ga:Zn = 4 × 10¹⁸ 4 × 10¹⁸ 4 × 10¹⁸apparatus A 1:1:0.5 (atomic ratio) atoms/cm³ atoms/cm³ atoms/cm³(In2O3:Ga2O3:ZnO = 1:1:1 (molar ratio)) In:Ga:Zn = 4 × 10¹⁸ 4 × 10¹⁸ 4 ×10¹⁸ 1:1:1 (atomic ratio) atoms/cm³ atoms/cm³ atoms/cm³ (In2O3:Ga2O3:ZnO= 1:1:2 (molar ratio)) Sputtering In:Ga:Zn = 4 × 10¹⁸ 4 × 10¹⁸ 4 × 10¹⁸apparatus B 1:1:0.5 (atomic ratio) atoms/cm³ atoms/cm³ atoms/cm³(In2O3:Ga2O3:ZnO = 1:1:1 (molar ratio)) In:Ga:Zn = 4 × 10¹⁸ 4 × 10¹⁸ 4 ×10¹⁸ 1:1:1 (atomic ratio) atoms/cm³ atoms/cm³ atoms/cm³ (In2O3:Ga2O3:ZnO= 1:1:2 (molar ratio))

In Table 1, the values measured by secondary ion mass spectrometry(SIMS) represent the average values in the effective range of SIMS.

From Table 1, it was found that, in the case where heat treatment wasperformed at 550° C. or higher for 1 hour or more, the hydrogenconcentration in the oxide semiconductor layer was significantlydecreased.

In other words, the oxide semiconductor layer having a hydrogenconcentration of 6×10¹⁸ atoms/cm³ or less is an oxide semiconductorlayer which has been subjected to heat treatment at high temperature (at550° C. or higher for 1 hour or more).

Note that when the transistors having the structure shown in FIG. 2Awere manufactured with only the conditions of first heat treatmentchanged, the transistor (450° C.) including an oxide semiconductor layerhaving a hydrogen concentration of 1×0¹⁹ atoms/cm³ or more had thresholdvoltage shifted in a negative direction and had significantdeterioration in the BT test compared with the transistors (550° C.,600° C., and 650° C.) each including an oxide semiconductor layer havinga hydrogen concentration of less than 1×10¹⁹ atoms/cm³.

Note that as the oxide semiconductor layer, an oxide semiconductor layerformed by sputtering using an oxide semiconductor target in which theatom ratio of In to Ga and Zn was 1:1:0.5 (In₂O₃:Ga₂O₃:ZnO=1:1:1 (at amolar ratio)) was used.

Note that before the formation of the oxide semiconductor layer, thereduction of leakage from a deposition chamber, the reduction ofnitrogen on the inner wall of the deposition chamber, deposition onto adummy substrate, or the like was sufficiently performed, so that aninfluence of nitrogen on the oxide semiconductor layer was thoroughlyprevented.

According to the above, the electrical characteristics of a transistorwere found to be influenced by hydrogen in an oxide semiconductor layer.

Example 8

FIG. 16 is a photograph of a cross section of a thin film transistorincluding an In—Ga—Zn—O-based oxide semiconductor which is observed witha transmission electron microscope (TEM, H-9000-NAR manufactured byHitachi, Ltd., 300 kV).

The thin film transistor shown in FIG. 16 is a sample obtained in such amanner that an In—Ga—Zn—O-based oxide semiconductor layer having athickness of 50 nm was formed as an oxide semiconductor layer 1601,first heat treatment (at 650° C. for 1 hour) was performed in a nitrideatmosphere, a titanium layer 1604 having a thickness of 150 nm wasformed as a metal film, and further, second heat treatment (at 250° C.for 1 hour) was performed in a nitrogen atmosphere.

In FIG. 16, at the interface between the oxide semiconductor layer 1601and the titanium layer 1604, an indium-rich layer 1602 and a titaniumoxide layer 1603 can be detected.

Note that the indium-rich layer 1602 and the titanium oxide layer 1603were detected by a fast fourier transform mapping (FFTM) method.

It was found that, through the second heat treatment in the state wherethe titanium layer 1604 and the oxide semiconductor layer 1601 were incontact with each other, oxygen was extracted from the oxidesemiconductor layer and the titanium oxide layer 1603 was formed.

Further, it was found that a portion where oxygen was extracted in theoxide semiconductor layer 1601 became the indium-rich layer 1602 inwhich a crystal of indium was precipitated.

In this manner, through the second heat treatment in the state where thetitanium layer 1604 and the oxide semiconductor layer 1601 are incontact with each other, the titanium oxide layer 1603 can be formed.

Note that it is clear that a similar reaction occurs even when an oxidesemiconductor layer other than an In—Ga—Zn—O-based oxide semiconductorlayer is used because a reaction of this example is a reaction betweenoxygen and titanium (Ti).

Therefore, the second heat treatment is preferably performed at 250° C.or higher (the upper limit is not limited, but can be set to 300° C. orlower, 400° C. or lower, 500° C. or lower, 600° C. or lower, or 700° C.or lower).

Note that a material having low resistance (a film containing aluminumas its main component, a film containing copper as its main component,or the like) is preferably formed over the titanium layer 1604 becausethe wiring resistance can be reduced.

This application is based on Japanese Patent Application serial no.2009-281505 filed with Japan Patent Office on Dec. 11, 2009, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising: asubstrate; a gate electrode over the substrate; a gate insulating layerover the gate electrode; an oxide semiconductor layer over the gateinsulating layer; a pair of contact electrodes over and in contact withthe oxide semiconductor layer; and an insulating layer over the oxidesemiconductor layer and the pair of contact electrodes, wherein the pairof contact electrodes are in contact with a side surface of the oxidesemiconductor layer, wherein the oxide semiconductor layer comprises achannel formation region, and wherein a nitrogen concentration in thechannel formation region is 1×10²⁰ atoms/cm³ or less.
 2. Thesemiconductor device according to claim 1, further comprising a channelprotective layer over and in contact with the oxide semiconductor layer,wherein the pair of contact electrodes is over the oxide semiconductorlayer and the channel protective layer.
 3. The semiconductor deviceaccording to claim 1, wherein a hydrogen concentration in the oxidesemiconductor layer is 6×10¹⁸ atoms/cm³ or less.
 4. The semiconductordevice according to claim 1, wherein the oxide semiconductor layer is inan oxygen-excess state.
 5. The semiconductor device according to claim1, wherein the oxide semiconductor layer comprises indium, gallium, andzinc.
 6. The semiconductor device according to claim 1, wherein theinsulating layer is an oxide insulating layer and in contact with partof the oxide semiconductor layer between the pair of contact electrodes.7. The semiconductor device according to claim 1, wherein each of thepair of contact electrodes comprises a metal film.
 8. A semiconductordevice comprising: a substrate; an oxide semiconductor layer over thesubstrate; a pair of contact electrodes over and in contact with theoxide semiconductor layer; a gate insulating layer over the oxidesemiconductor layer and the pair of contact electrodes; a gate electrodeover the gate insulating layer; wherein the pair of contact electrodesare in contact with a side surface of the oxide semiconductor layer,wherein the oxide semiconductor layer comprises a channel formationregion, and wherein a nitrogen concentration in the channel formationregion is 1×10²⁰ atoms/cm³ or less.
 9. The semiconductor deviceaccording to claim 8, wherein a hydrogen concentration in the oxidesemiconductor layer is 6×10¹⁸ atoms/cm³ or less.
 10. The semiconductordevice according to claim 8, wherein the oxide semiconductor layer is inan oxygen-excess state.
 11. The semiconductor device according to claim8, wherein the oxide semiconductor layer comprises indium, gallium, andzinc.
 12. The semiconductor device according to claim 8, wherein thegate electrode overlaps with the pair of contact electrodes.
 13. Thesemiconductor device according to claim 8, wherein each of the pair ofcontact electrodes comprises a metal film.